Crystallization of an amorphous silicon layer on glass is a step in the process of making a thin film transistor (TFT) array for a flat panel display. Currently, in routine commercial practice, this is carried out by irradiating the amorphous silicon layer, deposited on a glass substrate, with repetitively pulsed ultraviolet (UV) laser radiation. In commercial practice, the UV laser radiation is typically supplied by an excimer or exciplex laser. UV radiation is strongly absorbed by silicon, providing efficient conversion of the UV radiation into heat needed to melt the amorphous silicon to initiate crystallization.
There are two problems in using excimer laser radiation for silicon crystallization. These problems are currently tolerated in the absence of any cost effective alternative to excimer laser radiation. One problem is the excimer lasers themselves. They are expensive to buy and maintain. The gain materials of the lasers include fluorine and chlorine, so that strict attention must be paid to gas containment. The lasers provide gain through pulsed discharges in the lasing gases at pressures greater than one atmosphere. Laser gases must be circulated through the discharges and through cryogenic cleaning systems that are used to keep the gases free of contaminants. The lasers and associated beam projection optics must be carefully vibration-isolated from electrical circuitry used to generate pulse trains for creating the gas discharges, and from motors and pumps used for gas circulation.
Another problem is that the radiation provided by an excimer laser is repetitively pulsed radiation, with the pulses having relatively short, for example tens of nanoseconds, duration. Pulses are delivered at a rate no greater than about 4 kilohertz (kHz). In an amorphous silicon layer exposed to one pulse, amorphous silicon melts, silicon crystals begin to grow in the melt, and then the silicon solidifies before the delivery of the next pulse. The silicon crystals created in a single pulse are too small to provide adequate carrier mobility in a TFT, and crystallization is incomplete. Because of this, exposure to multiple pulses, providing multiple melting a solidifying cycles, is necessary. This causes re-crystallization wherein some crystals provided by one pulse survive and grow and eventually absorb smaller crystals produced by subsequent pulses.
There have been laboratory experiments performed using pulsed radiation having a wavelength of about 532 nm, i.e., radiation in the green region of the visible spectrum, from frequency-doubled solid state lasers such as frequency doubled neodymium doped YAG and yttrium vanadate (Nd:YAG and Nd:YVO4) lasers. These pulses have a typically longer duration. These experiments illustrate, inter-alia, that crystallization is possible at wavelengths that are less strongly absorbed by the silicon than ultraviolet radiation without compromising quality of the crystallization. There still remains a problem that pulsed radiation is needed to provide adequate power.
It has believed that if about 100 Watts or more of continuous wave (CW) radiation having a wavelength in the green or blue spectral region could be concentrated in a line of light having a length of a few millimeters (mm) and a width less than about five micrometers (μm), there would be sufficient radiation intensity in the line of light to quickly melt silicon. The line of light could be scanned over an area of amorphous silicon to be crystallized such that melting and subsequent growth of crystals of adequate size could take place during the dwell time of the line of light on the amorphous layer, i.e., without a need for re-crystallization. Scanning the line would also allow an extended area of amorphous silicon to be crystallized. The intensity of radiation along the line, however, would need to be substantially uniform along the line, for example, within about 5%, or even less, of a nominal value.
CW blue and green radiation can be generated with about 25% or greater electrical-to-optical efficiency by intracavity frequency doubling near infrared (NIR) radiation in an external-cavity, optically pumped (diode-pumped), surface-emitting, semiconductor laser. Such a laser is usually designated an OPS-laser by practitioners of the art. Such a laser includes what is commonly termed an OPS-structure. The OPS-structure includes a mirror structure surmounted by a multilayer surface-emitting semiconductor gain structure. The mirror structure usually functions as an end-mirror of a laser resonator (cavity) in the which the gain structure is located. At a present state of development, such a laser having an average lifetime of five thousand hours or more between services and having a green (second-harmonic) output power between about 10 W and 15 W in a single or a few modes, is available from the Coherent Inc. of Santa Clara, Calif., the assignee of the present invention. Such a laser can have a beam quality defined by an M2 value of about 10 or less. This beam quality would be suitable for projecting an image having a dimension of 5 μm or less. The power output, however, falls well short of what would be needed to provide the target power in a 5 mm×5 μm line.
A primary limit to the output power of an OPS laser is presented by the difficulty of removing heat (from unconverted pump light) from the OPS gain-structure. This limits the pump power that can be applied to a single structure. Proposals have been made to overcome this limitation by including more than one gain-structure in a resonator. One such approach is described in U.S. patent application Ser. No. 11/124,319 filed, May 6, 2005. Practical problems associated with such an approach include matching the gain bands of the gain-structures, and configuring the resonator such that a change in the condition of one OPS-structure, in particular, the flatness of the mirror structure, does not adversely influence the power available from one or more of the other gain-structures. While these problems may eventually be solved, it remains to be seen whether or not the solution of the problems will cause the cost scaling of a multi-gain-structure OPS-laser to exceed the power scaling achieved.
It is believed that CW, visible-wavelength, amorphous silicon crystallization could be made commercially viable if an OPS-laser-based radiation delivery system can be developed that provides adequate laser power without a need to solve problems associated with power scaling individual OPS-lasers. The system, of course, must provide the necessary power while still having sufficient optical quality to project a line having a width of about 5 μm or less.